I've never protected holly in the winter, but if they are in pots you could move them to a sheltered spot, especially if you are in a colder area, until they are growing strongly.
Different types of holly grow at different rates, but they all take time to get used to new conditions, and very much resent change. After the first season, growth rate should increase. Hollies can grow up to 40 feet in height, but a dwarf one would stop at about 4 ft. Mine have always preferred sun and not too much water, and once they are established have put on enough growth to fill the house in vases ! But I think you are a little optimistic about decorating with them this year :o).
I still have no idea why one has darker leaves but it may just be the plant's natural tendency.